Antibody drug conjugates (ADCs), a rapidly growing class of targeted therapeutics, represent a promising new approach toward improving both the selectivity and the cytotoxic activity of cancer drugs. An example of ADC drug having been approved for therapeutic use in the U.S. is brentuximab vedotin (ADCETRIS®), a chimeric anti-CD30 antibody conjugated to monomethyl auristatin E used in treating anaplastic large cell lymphoma and Hodgkin lymphoma.
One conventional method employed in the design of antibody-drug conjugates (ADC) involves the coupling of drug molecules to thiol groups of antibody chains via a linking moiety. Free thiol groups are obtained as a result of breaking the cysteine interchain disulfide bonds of an antibody via a reduction reaction. A typical antibody contains 4 interchain disulfide bonds (2 between the heavy chains and 2 between the heavy and light chains). These interchain disulfides can be selectively reduced with dithiothreitol, tris(2-carboxyethyl)phosphine, or other mild reducing agents, to result in 8 reactive sulfhydryl groups for conjugation. This method can link up to eight drug molecules to a given antibody.
Due to the fact that at least two disulfide bonds are broken, ADCs designed using this principle are unstable once entered into circulation, and thus the half life of ADCs will be shortened. As a result, recent development in ADC design and synthesis adopts a different approach, namely, one that relies on covalently connecting two thiol groups by a coupling agent, thereby establishing thiol bridges between the two heavy chains and between the heavy and the light chains of a given antibody. Current research efforts exploring such an approach mainly focus on designing the structure of a coupling agent that not only has the functionalities to bridge two thiol groups covalently, but also encompasses the necessary components to facilitate specific biological activities.
Earlier studies in the field have utilized bis-maleimides to react with the two thiol groups resulted from a broken disulfide bond. The covalent coupling between maleimides and thiols is a classic alkene conversion reaction. More recently studied thiol-bridging reactions are also based upon this principle, with exemplary reactions involving maleimides, bis-maleimides, and maleimides with halogen substituents. To date, however, thiol-bridging linker compositions are limited to maleimides-based compounds only, and oftentimes not specified for applications in tumor-targeting ADCs. For example, previously disclosed methods of covalent thiol-coupling involving the use of similar maleimides-based compounds did not specify their applications in coupling with active agents such as tumor-targeting drug molecules, proteins, or polypeptides (see, for example, PCT Patent Application Publication No. WO 2013132268). Other methods that specifically disclosed applications in tumor-targeting ADCs utilizing maleimides-based compounds did not employ the covalent thiol-bridging mechanism in which one linker simultaneously reacts with two thiol groups, as provided by the present invention (see, for example, Chinese Patent Application Publication No. CN 103933575).